Final answer:
The substance with the largest molar entropy at 298K among the given options is b) Cl₂ (g) due to its gaseous state and larger size compared to F₂ (g), while both being more disordered than the liquid Br₂ and the solid I₂.
Step-by-step explanation:
The student has asked which of the following substances has the largest molar entropy at 298K: I₂ (s), Cl₂ (g), F₂ (g), Br₂ (g), or Br₂ (l). To answer this question, we must consider the physical state of each substance at room temperature, as entropy is closely related to the amount of disorder in a system. At room temperature, Cl₂ and F₂ are gases, Br₂ is a liquid, and I₂ is a solid.
Entropy generally increases from solids to liquids to gases, because gases have the most disorder and freedom of movement of molecules. As such, the gas Cl₂ and F₂ will have higher entropy than the liquid Br₂, and all of these will have higher entropy than the solid I₂. Between Cl₂ and F₂, the difference in entropy will depend on molecular complexity and mass. However, since both Cl₂ and F₂ are diatomic molecules of similar complexity, considering the physical state alone, Cl₂ (g) ultimately has the largest molar entropy at 298K due to its larger size and thus a larger number of accessible microstates compared to F₂ (g).
Hence, the correct answer is Option B.